Experienced by a thick-walled vessel due to the combined stresses from a rapid temperature and/or pressure change. Non-uniform temperature distribution and subsequent differential expansion and contraction are the causes of the stresses involved
Pressurized thermal shock
Identify the three components that will set limits on the heatup and cooldown rates
Flanges
Reactor Vessel Head
Reactor Vessel
Identify the basis for determining heatup and cooldown rate limits
Failure Life - impact on the life of the plant prior to fatigue becoming a likely failure made
Limiting thermal stress - created when the large components of the reactor vessel are heated and cooled
Given a minimum pressure and temperature (MPT) graph, determine the limiting temperature or pressure.
Minimum Bolt up 70oF
Given a minimum pressure and temperature graph determine the minimum temperature allowed to tension the vessel head and state basis
80oF
Most limiting temperature based on most limiting component
State the action typically taken upon discovering the heatup or cooldown rate has been exceeded.
No immediate hazard, only requires an assessment of the impact on the future fatigue life of the plant
State the reason for using soaktimes
So that heating can be carefully controlled. In this manner thermal stresses are minimized
State when soak times become significant
When a limiting component is at room temperature or below and very close to its RTNDT temperature limitations
Identify two stresses that are the result of thermal shock to plant materials
Tensile stress
Compressive stress
State the two causes of thermal stress
Non-uniform heating of a uniform metal
Uniform heating of a non-uniform material
Describe why thermal shock is a major concern in reactor system when rapidly heating or cooling a thick walled vessel
Concern due to the magnitude of the stresses involved with rapid heating or cooling
List three operational limits that are specifically intended to reduce the severity of the thermal shock
Heatup and Cooldown rate limits
Temperature limits for placing systems into operation
Specific temperatures for specific pressures for system operations
STATE how the pressure in a closed system affects the severity of thermal shock
Pressure raises the severity due to additive effect of thermal and pressure tensile stresses
List the four plant transients that have the greatest potential for causing thermal shock
Excessive heatup and cooldown
Plant Scrams
Plant pressure excursions outside of normal pressure bands
Loss of coolant accident (LOCA)
State the three locations of primary concern for thermal shock in a reaction system